1. Field of the Invention
This invention relates to a digital video mapping device for use in radar data processing systems and particularly to an infinite ratio clutter detector and mapper that automatically rejects fading or intermittent clutter.
2. Description of the Prior Art
In a radar system which automatically processes video from a surveillance radar, and distinguishes between valid and invalid targets, the automatic data processing system attempts to identify valid targets and when a specified percentage of hits are found a target indication is reported. One of the principal problems involved in determining the validity of the targets results from the presence of clutter which may come from ground structures, bodies of water or from weather conditions. An example of an automatic clutter mapper system is shown in U.S. Pat. No. 3,325,806 which rejected stationary clutter by counting target detections in each clutter mapper cell. This technique satisfactorily allowed the rejection of stationary clutter returns and in the presence of stationary clutter provided a substantially constant clutter false alarm rate. The rejection of stationary clutter returns which were scintillating so that they were not detected every scan is provided by the system in the U.S. Pat. No. 3,720,942 which utilizes a weighted up-down counter to keep a record of detections in each cell. The system of this patent includes a counter with a weighting value of four for detection and a one for a miss so the clutter returns that occurred in a cell more than about 25% of the time were rejected. In these previous techniques, clutter that occurred in an area clutter mapping cell less than 10% of the time such as intermittent clutter or fading clutter is not satisfactorily detected and the clutter false alarm rate conditions are not satisfied under this atmospheric condition. It is found that a clutter condition whereby a number of inversion layers are present in the atmosphere causes the radar beam to bend by refraction to reflect the clutter over the horizon with the refraction varying so that clutter occurs only a small percentage of the time or in an intermittent fashion. It would be a substantial advantage of the art if a clutter mapping system were provided that would satisfactorily detect the presence of not only stationary clutter but intermittent or fading clutter that occurs a small percentage of the time in any quantum reception area, so that the presence of invalid targets resulting from clutter could be reliably determined.